The present invention relates to a fuel to be used for a fuel cell system.
Recently, with increasing awareness of the critical situation of future global environments, it has been highly expected to develop an energy supply system harmless to the global environments. Especially urgently required are to reduce CO2 to prevent global warming and reduce harmful emissions such as THC (unreacted hydrocarbons in an exhaust gas), NOx, PM (particulate matter in an exhaust gas: soot, unburned high boiling point and high molecular weight fuel and lubricating oil). Practical examples of such a system are an automotive power system to replace a conventional Otto/Diesel engine and a power generation system to replace thermal power generation.
Hence, a fuel cell, which has high energy efficiency and emits only H2O and CO2, has been regarded as a most expectative system to response to respond to social requests. In order to achieve such a system, it is necessary to develop not only the hardware but also the optimum fuel.
Conventionally, as a fuel for a fuel cell system, hydrogen, methanol, and hydrocarbons have been candidates.
As a fuel for a fuel cell system, hydrogen is advantageous in a point that it does not require a reformer, however, because of a gas phase at a normal temperature, it has difficulties in storage and loading in a vehicle and special facilities are required for its supply. Further, the risk of inflammation is high and therefore, it has to be handled carefully.
On the other hand, methanol is advantageous in a point that it is relatively easy to reform, however power generation quantity per weight is low and owing to its toxicity, handling has to be careful. Further, it has a corrosive property, special facilities are required for its storage and supply.
Like this, a fuel to sufficiently utilize the performances of a fuel cell system has not yet been developed. Especially, as a fuel for a fuel cell system, the following are required: power generation quantity per weight is high; power generation quantity per CO2 emission is high; a fuel consumption is low in a fuel cell system as a whole; an evaporative gas (evapo-emission) is a little; deterioration of a fuel cell system comprising such as a reforming catalyst, a water gas shift reaction catalyst, a carbon monoxide conversion catalyst, fuel cell stacks and the like is scarce to keep the initial performances for a long duration; a starting time for the system is short; and storage stability and handling easiness are excellent.
Incidentally, in a fuel cell system, it is required to keep a fuel and a reforming catalyst at a proper temperature, the net power generation quantity of the entire fuel cell system is equivalent to the value calculated by subtracting the energy necessary for keeping the temperature (the energy for keeping balance endothermic and exothermic reaction following the preheating energy) from the actual power generation quantity. Consequently, if the temperature for the reforming is lower, the energy for preheating is low and that is therefore advantageous and further the system starting time is advantageously shortened. In addition, it is also necessary that the energy for preheating per fuel weight is low. If the preheating is insufficient, unreacted hydrocarbon (THC) in an exhaust gas increases and it results in not only decrease of the power generation quantity per weight but also possibility of becoming causes of air pollution. To say conversely, when some kind of fuels are reformed by the same reformer and the same temperature, it is more advantageous that THC in an exhaust gas is lower and the conversion efficiency to hydrogen is higher.
The present invention, taking such situation into consideration, aims to provide a fuel suitable for a fuel cell system satisfying the above-described requirements in good balance.
Inventors of the present invention have extensively investigated to solve the above-described problems and found that a fuel comprising a specific amount of oxygenates (oxygen-containing compounds) is suitable for a fuel cell system.
That is, a fuel for a fuel cell system according to the invention comprises;
(1) 0.5-40 mass % of oxygenates therein in terms of an oxygen content based on the whole fuel.
The fuel comprising the specific amount of the oxygenates is preferable to satisfy the following additional requirements;
(2) the fuel contains 5 vol. % or more of hydrocarbons based on the whole fuel;
(3) a content of hydrocarbon compounds having a carbon number of 4 is 15 vol. % or less, the content of hydrocarbon compounds having a carbon number of 5 is 5 vol. % or more and the content of hydrocarbon compounds having a carbon number of 6 is 10 vol. % or more based on the whole hydrocarbons;
(4) distillation properties are the initial boiling point in distillation of 24xc2x0 C. or higher and 40xc2x0 C. or lower, the 10 vol. % distillation temperature of 25xc2x0 C. or higher and 50xc2x0 C. or lower, the 90 vol. % distillation temperature of 45xc2x0 C. or higher and 130xc2x0 C. or lower, and the final boiling point in distillation of 55xc2x0 C. or higher and 150xc2x0 C. or lower;
(5) a sulfur content is 50 ppm by mass or less based on the whole fuel;
(6) saturates are 30 vol. % or more based on the whole hydrocarbons;
(7) olefins are 35 vol. % or less based on the whole hydrocarbons;
(8) aromatics are 50 vol. % or less based on the whole hydrocarbons;
(9) a ratio of paraffins in saturates is 60 vol. % or more;
(10) a ratio of branched paraffins in paraffins is 30 vol. % or more;
(11) heat capacity of the fuel is 2.6 kJ/kg xc2x0 C. or less at 15xc2x0 C. and 1 atm in liquid phase;
(12) heat of vaporization is 400 kJ/kg or less;
(13) Reid vapor pressure (RVP) is 10 kPa or more and less than 100 kPa;
(14) research octane number (RON, the octane number by research method) is 101.0 or less;
(15) oxidation stability is 240 minutes or longer; and
(16) density is 0.78 g/cm3 or less.